1. Technical Field of the Invention
The present invention relates to a ferroelectric memory device, and particularly to a circuit and method for providing a ferroelectric memory device having reduced imprint effects.
2. Background of the Invention
Ferroelectricity is a phenomenon which can be observed in a relatively small class of dielectrics called ferroelectric materials. In a normal dielectric, upon the application of an electric field, positive and negative charges will be displaced from their original position--a concept which is characterized by the dipole moment or polarization. This polarization or displacement will vanish, however, when the electric field returns back to zero. In a ferroelectric material, on the other hand, there is a spontaneous polarization--a displacement which is inherent to the crystal structure of the material and does not disappear in the absence of the electric field. In addition, the direction of this polarization can be reversed or reoriented by applying an appropriate electric field.
These characteristics result in ferroelectric capacitors, formed from ferroelectric film or material disposed between parallel conduction plates, being capable of storing in a nonvolatile manner a first charge corresponding to a first polarization state in which the direction of polarization is in a first direction, and a second charge corresponding to a second polarization state in which the direction of polarization is in a second direction opposite the first direction. Ferroelectric capacitors are utilized in nonvolatile random access memory devices having a memory cell array architecture that is similar to the memory cell array architecture of dynamic random access memory (DRAM) devices.
In general terms, there are two types of ferroelectric memory cells. Referring to FIG. 1A, a one transistor, one capacitor (1T1C) memory cell utilizes a pass gate transistor T connected between a bit line B and a first plate of ferroelectric capacitor C. A second plate of ferroelectric capacitor C is connected to a plate line P. The gate terminal of pass gate transistor T is connected to a word line W. A memory device utilizing a 1T1C memory cell uses a reference memory cell that is accessed at the same time the 1T1C memory cell is accessed so as to provide a charge differential appearing across a pair of bit lines coupled to the 1T1C cell and the reference cell. The use of 1T1C ferroelectric memory cells is known in the art.
Referring to FIG. 1B, a two transistor, two capacitor (2T2C) memory cell includes two ferroelectric capacitors C1 and C2. A first pass gate transistor T1 is connected between a first plate of ferroelectric capacitor C1 and a first bit line BL of a bit line pair. A second pass gate transistor T2 is connected between a first plate of ferroelectric capacitor C2 and a second bit line BL' of the bit line pair. A second plate of ferroelectric capacitors C1 and C2 is connected to a plate line P. The gate terminal of pass gate transistors T1 and T2 is connected to the word line W. Each capacitor C1 and C2 stores a charge representative of the polarization state thereof, the charge combining with the charge of the other capacitor to result in a charge differential appearing across bit lines BL and BL' when the 2T2C memory cell is accessed. The polarity of the charge differential denotes the binary value stored by the 2T2C memory cell. The use of 2T2C ferroelectric memory cells is known in the art.
A problem with ferroelectric memory devices is the existence of a phenomenon known as imprint. Imprint is a characteristic of ferroelectric films that refers to the tendency of a ferroelectric film/capacitor to prefer one polarization state over another polarization state. Imprint is known to occur when a ferroelectric capacitor is maintained in a single polarization state for a prolonged period of time. Imprint adversely effects the ability of a ferroelectric capacitor to switch between the polarization states. Consequently, the existence of imprint may directly impact the performance of a ferroelectric memory device.